Dual-mode/function optical and electrical interconnects, methods of fabrication thereof, and methods of use thereof

ABSTRACT

Devices and systems having one or more of the following components: a compliant pillar with a modified tip surface (non-flat tip) and a corresponding compliant socket; an optical/electrical I/O interconnect and a corresponding compliant socket; a lens/waveguide optical pillar, a polymer bridge, and an L-shaped pillar, are described herein. In addition, methods of making these components and methods of using these components are disclosed herein.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. provisional applicationentitled, “COMPLIANT ELECTRICAL, OPTICAL AND RF POLYMER PILLARS ANDPOLYMER CONNECTORS,” having serial No. 60/405,934, filed on Aug. 26,2002 and U.S. provisional application entitled, “OPTICAL AND ELECTRICALI/O INTERCONNECT FABRICATION AND CONFIGURATIONS,” having serial No.60/457,381, filed on Mar. 25, 2003, which are both entirely incorporatedherein by reference. This application is related to co-pending U.S.nonprovisional applications entitled, “COMPLIANT WAFER-LEVEL PACKAGESWITH PILLARS AND METHODS OF FABRICATION,” having Ser. No. 10/285,034,filed Oct. 31, 2002, and “DEVICES HAVING COMPLIANT WAFER-LEVELINPUT/OUTPUT INTERCONNECTIONS AND PACKAGES USING PILLARS AND METHODS OFFABRICATION THEREOF,” having Ser. No. 10/430,670, filed on May 5, 2003,which are both entirely incorporated herein by reference.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT

[0002] The U.S. government may have a paid-up license in this inventionand the right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofMDA972-99-1-0002 awarded by the DARPA.

TECHNICAL FIELD

[0003] The present invention is generally related to integratedcircuits, optoelectronics, photonics, waveguides, optical waveguidesand, more particularly, is related to devices having non-flat pillartips, dual-mode/function input/output interconnections, and packagingthereof, methods of fabrication thereof, and methods of use thereof.

BACKGROUND OF THE INVENTION

[0004] Conventional chip manufacturing is divided into front-end,back-end, and tail-end processing. Front-end of the line (FEOL)processing refers to the fabrication of transistors, while back-end ofthe line (BEOL) processing describes wafer metallization. Tail-end ofthe line (TEOL) processing refers to the packaging of the individualdice. Generally, the final wafer-level process step is the fabricationof vias through a passivation layer to expose the die pads, which serveas the interface between the die and the package. Each individual die,while still part of the wafer, is then functionally tested for wafersort. The dice that pass this test are shipped to a packaging foundrywhere they are individually placed in a temporary package for burn-in.These dice are then individually packaged into their final package andtested again for functionality. This final step concludes tail-endprocessing and the functional packaged dice are ready for systemassembly.

[0005] The mechanical performance of a package is important forwafer-level testing, protection, and reliability. Wafer-level testing ofelectrical devices requires simultaneous reliable electrical contactacross a surface area. Typically, neither the wafer nor the testingsubstrate is planar enough to enable this reliable temporary electricalcontact. In-plane (i.e., x-, y- axis) compliance is generally requiredto account for potential problems such as, for example, thermalexpansion mismatch between the chip and printed wiring board and theprobe contact leads. Wafer-level testing and burn-in demand significantout-of-plane (i.e., z-axis) compliance in order to establish reliableelectrical contact between the pads on the non-planar wafer andpads/probes on the board surfaces. Non-compliance of the input/output(I/O) interconnects/pads out-of-plane, as well as in-plane (i.e., x-, y-axis), can cause difficulties in performing wafer-level testing and poorreliability. For optical interconnection, the alignment between the chipand the board should be maintained during field service to minimizeoptical losses due to offset.

[0006] A key interconnection level that will be severely challenged bygigascale integration (GSI) is the chip-to-module interconnection thatintegrates the packaged chip into the system. A gigascalesystem-on-a-chip (SoC) demands the development of new and cost-effectiveintegrated input/output (I/O) interconnect solutions that usehigh-performance integrated electrical, optical, and radio frequency(RF) approaches to meet all of the I/O requirements of the 45 to 22 nmInternational Technology Roadmap for Semiconductors (ITRS) technologynodes (International Technology Roadmap for Semiconductors (ITRS), 2002update, SIA). Meeting these challenges is essential for thesemiconductor industry to transcend known limits on interconnects thatwould otherwise decelerate or halt the historical rate of progresstoward GSI and beyond. In general, power, clock, and signal I/Ofunctions will require the selective integration of fine pitchelectrical (<30 μm pitch area array), optical, and RF I/O interconnecttechnologies. These high-density integrated I/O are needed for novel 3Dstructures as well as for high current (>400A) and high bandwidth (>40Tbs) applications. To solve the above issues it is required to overcomelong-range and fundamental barriers in chip-to-module interconnects byadvancing fine-pitch compliant interconnections, optoelectronic and RFinterconnections, and wafer-level testing and burn-in.

[0007] Accordingly, there is a need in the industry to address theaforementioned deficiencies and/or inadequacies.

SUMMARY OF THE INVENTION

[0008] Embodiments of the present invention include devices havinginput/output (I/O) interconnect systems. A representative I/Ointerconnect system includes a first substrate having at least onecompliant pillar transversely extending from the first substrate. Thecompliant pillar includes a first material. In addition, the compliantpillar includes a non-flat tip at the end opposite the first substrate.

[0009] The present invention provides for a method of fabricating adevice having at least one compliant pillar. The method includesproviding a substrate, disposing a material onto at least one portion ofthe substrate, and removing portions of the material to form at leastone pillar on the substrate.

[0010] In addition, the present invention provides for a dual-modeoptical/electrical input/output (I/O) interconnect system. Arepresentative optical/electrical I/O interconnect includes a firstsubstrate having at least one optical/electrical I/O interconnect thatincludes a pillar transversely extending from the first substrate. Thepillar comprises a first material, which is optically conductive. Thepillar also includes a lead disposed over a portion of the pillarextending from the base of the pillar on the first substrate to the endopposite the first substrate.

[0011] The present invention also provides methods for fabricating suchdevices. A representative method, among others, can be summarized by thefollowing steps: providing a first substrate having at least oneoptical/electrical I/O interconnect that includes a pillar transverselyextending from the first substrate, wherein the pillar includes a firstmaterial, the first material is optically conductive, and the pillarincludes a lead disposed over a portion of the pillar extending from thebase of the pillar on the first substrate to the end opposite the firstsubstrate; providing a second substrate having at least one socketadapted to receive the optical/electrical I/O interconnect, wherein thesocket includes a second material, wherein the second substrate includesa lead contact that communicatively connects the first substrate and thesecond substrate through the lead and an optical contact thatcommunicatively connects the first substrate and the second substratethrough the pillar; and causing the socket to receive a portion of theoptical/electrical I/O interconnect.

[0012] In addition, the present invention provides for a method ofdirecting optical energy and electrical energy simultaneously. Themethod can be broadly conceptualized as follows: providing a firstsubstrate having at least one optical/electrical I/O interconnect thatincludes a pillar transversely extending from the first substrate,wherein the pillar includes a first material, the first material isoptically conductive, and the pillar includes a lead disposed over aportion of the pillar extending from the base of the pillar on the firstsubstrate to the end opposite the first substrate; providing a secondsubstrate having at least one socket adapted to receive theoptical/electrical I/O interconnect, wherein the socket comprises asecond material, wherein the second substrate includes a lead contactthat communicatively connects the first substrate and the secondsubstrate through the lead and an optical contact that communicativelyconnects the first substrate and the second substrate through thepillar; communicating optical energy through the pillar of the firstsubstrate to the optical contact of the second substrate; andcommunicating electrical energy through the lead of the first substrateto the lead contact of the second substrate.

[0013] Furthermore, the present invention provides for a method forforming a device. A representative method, among others, can besummarized by the following steps: providing a first substrate having atleast one optical/electrical I/O interconnect that includes a pillartransversely extending from the first substrate, wherein the pillarcomprises of a first material, the first material is opticallyconductive, and the pillar includes a lead disposed over a portion ofthe pillar extending from the base of the pillar on the first substrateto the end opposite the first substrate; providing a second substratehaving at least one socket adapted to receive the optical/electrical I/Ointerconnect, wherein the socket comprises a second material, whereinthe second substrate includes a lead contact that communicativelyconnects the first substrate and the second substrate through the lead,wherein the second substrate includes an optical contact thatcommunicatively connects the first substrate and the second substratethrough the pillar; and causing the socket to receive a portion of theoptical/electrical I/O interconnect.

[0014] Furthermore, the present invention provides for a method ofaligning substrates. A representative method, among others, can besummarized by the following steps: providing a first substrate having atleast one optical/electrical I/O interconnect that includes a pillartransversely extending from the first substrate, wherein the pillarcomprises of a first material, the first material is opticallyconductive, and the pillar includes a lead disposed over a portion ofthe pillar extending from the base of the pillar on the first substrateto the end opposite the first substrate; providing a second substratehaving at least one socket adapted to receive the optical/electrical I/Ointerconnect, wherein the socket comprises a second material, whereinthe second substrate includes a lead contact that communicativelyconnects the first substrate and the second substrate through the lead,wherein the second substrate includes an optical contact thatcommunicatively connects the first substrate and the second substratethrough the pillar; maintaining optical alignment between the firstsubstrate and the second substrate using the optical/electrical I/Ointerconnect and the socket; and maintaining electrical interconnectionbetween the first substrate and the second substrate using theoptical/electrical I/O interconnect and the socket.

[0015] Other systems, methods, features, and advantages of the presentinvention will be, or become, apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Many aspects of the disclosed devices and methods can be betterunderstood with reference to the following drawings. The components inthe drawings are not necessarily to scale, emphasis instead being placedupon clearly illustrating the relevant principles. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

[0017]FIG. 1A illustrates a cross-sectional view of a representativeembodiment of an input/output (I/O) interconnection system, while FIG.1B illustrates cross-sectional views of the non-flat compliant pillar(cross section a-a of FIG. 1A) and the compliant socket (cross sectionb-b of FIG. 1A).

[0018]FIGS. 2A through 2I are lateral cross-sectional views ofrepresentative compliant pillars having a plurality of exemplarycross-sections.

[0019]FIGS. 3A and 3B are cross-sectional views of representativecompliant sockets having a plurality of exemplar cross-sections.

[0020]FIGS. 4A through 4F are cross-sectional views that illustrate arepresentative process for fabricating the non-flat compliant pillarillustrated in FIGS. 1A and 1B.

[0021]FIGS. 5A through 5F are cross-sectional views that illustrate arepresentative process for fabricating the compliant socket illustratedin FIGS. 1A and 1B.

[0022]FIG. 6 illustrates a cross-sectional view of a representativeembodiment of a dual optical/electrical I/O interconnection system.

[0023]FIGS. 7A through 7F are cross-sectional views that illustrate arepresentative process for fabricating the dual optical/electrical I/Ointerconnection system illustrated in FIG. 6.

[0024]FIG. 8 illustrates a cross-sectional view of a representativeembodiment of a pillar having a non-flat modified-tip.

[0025]FIGS. 9A through 9J are cross-sectional views that illustrate arepresentative process for fabricating the non-flat modified pillarillustrated in FIG. 8

[0026]FIG. 10 illustrates a cross-sectional view of optical energytraversing through the modified pillar of FIG. 8.

[0027]FIGS. 11A and 11B illustrate cross-sectional views ofrepresentative embodiments of a pillar having a lens disposed of the tipof the pillar.

[0028]FIGS. 12A through 12F are a cross-sectional view that illustrate arepresentative process for fabricating the pillar illustrated in FIG.11A.

[0029]FIG. 13 illustrates a cross-sectional view of optical energytraversing through the pillar of FIG. 11A.

[0030]FIG. 14A illustrates a cross-sectional view of a representativeembodiment of a polymer bridge, while FIG. 14B illustrates anothercross-sectional view of the polymer bridge (cross section a-a of FIG.14A).

[0031]FIGS. 15A through 15C illustrate cross-sectional views ofrepresentative embodiments of polymer bridges.

[0032]FIGS. 16A through 16D illustrate cross-sectional views of arepresentative process for fabricating the polymer bridge illustrated inFIG. 14A.

[0033]FIGS. 17A and 17B illustrate cross-sectional views ofrepresentative embodiments of L-shaped pillars.

[0034]FIGS. 18A and 18B illustrate representative top views of L-shapedpillars shown in FIGS. 17A and 17B.

[0035]FIGS. 19A and 19E are cross-sectional views that illustrate arepresentative process for fabricating the L-shaped pillar illustratedin FIG. 17A.

DETAILED DESCRIPTION

[0036] Devices and systems having one or more of the followingcomponents: a compliant pillar with a modified tip surface (non-flattip) and a corresponding compliant socket; an optical/electrical I/Ointerconnect and a corresponding compliant socket; a lens/waveguideoptical pillar, a polymer bridge, and a L-shaped pillar, are describedherein. In addition, methods of making these components and methods ofusing these components are disclosed herein.

[0037] The types of devices that can use the compliant pillar, theoptical/electrical I/O interconnect, the lens/waveguide optical pillar,the polymer bridge, the L-shaped pillar and their correspondingcompliant sockets include, but are not limited to, high speed and highperformance chips such as, but not limited to, microprocessors,communication chips, and optoelectronic chips.

[0038] The components can be fabricated of one or more materials thatenhance compliance in-plane and out-of-plane (i.e., x-, y- axis andz-axis directions, respectively). The fabrication of the components withthis material allows the components to be compliant in the x-, y- and/orz directions, which allows the components to be attached to a chipand/or printed board with a higher coefficient of thermal expansionwithout underfill, thus lowering costs and enhancing reliability. Inaddition, forming the compliant pillar, the optical/electrical I/Ointerconnect, the lens/waveguide optical pillar, and/or thecorresponding compliant sockets, on a polymer bridge enhances compliancein the z-axis direction.

[0039] The compliant pillar, the optical/electrical I/O interconnect,the lens/waveguide optical pillar, and the L-shaped pillar are disposedsubstantially transversely (i.e., substantially vertical to thesubstrate) to the substrate as shown in the FIGS. 1A, 1B, 6, 8, 11A,11B, 17A, and 17B.

[0040] For optical interconnection, alignment should be maintainedbetween the optical devices on the board and the chip during fieldservice. As a result, it is important to mitigate the offset that may beintroduced by thermal expansion mismatches between the chip and theboard. This problem can, at least in part, be solved with a mechanicallyflexible (compliant) optical waveguide pillar that is perpendicular tothe chip on which it is disposed. This mitigates optical losses due tooffset. In addition, the optical waveguide pillars prevent lightspreading as it is routed between two parallel surfaces, such as chipand a board.

[0041] Dual optical/electrical I/O interconnects allow for a singlepillar to be used as a platform to communicate both optical energy(pillar waveguide) and electrical energy (electrical lead) energy orradio frequency (RF) energy (RF lead). The optical/electrical I/Ointerconnect can guide optical energy from a first substrate to a secondsubstrate positioned substantially horizontal (e.g., substantially inthe same plane as) to the first substrate, while also connecting anelectrical signal via the lead from the first substrate to the secondsubstrate. In one embodiment, the tip of the pillar can be a non-flat(e.g., slanted) surface and the metal from the lead can be disposed overthe slanted portion of the tip. In this manner, the metal can be used asa mirror to direct the optical energy. In another embodiment, an element(e.g., grating coupler or mirror) can be used at the tip of the pillarto guide the optical energy. In still another embodiment, a mirror orgrating coupler can be used on the second substrate to guide the opticalenergy out of the pillar.

[0042] Dual optical/electrical I/O interconnects are advantageous for atleast the following reasons: 1) the same space is being used forelectrical/optical I/O, and thus, there is a very high density ofinterconnections, 2) there is very high interconnect process integrationbetween the two, 3) they can be made compliant, 4) they maintainalignment, 5) they can be wafer-level batch fabricated, and 6) thesockets aid in attachment.

[0043] In addition, the use of these components enables ultra high I/Odensity (e.g., about 10 to about 500,000 or more components percentimeter squared (cm²)) to be achieved on the chip at wafer-level andprinted board, which can enhance power distribution, increase I/Obandwidth, satisfy three-dimensional structural I/O demands, suppresssimultaneous switching noise, improve isolation in mixed signal systems,and decrease costs. In addition, wafer-level functionality testing aswell as wafer-level burn-in, which can be used to identify known goodpackaged die (KGPD), can be enhanced (e.g., reduced time and cost).Furthermore, for optical and RF interconnections, high density I/Osenable massive chip to board bandwidth.

[0044] The compliant pillar, the opticavelectrical I/O interconnect, thelens/waveguide optical pillar, the polymer bridge, and the L-shapedpillar, can be batch-fabricated at the wafer level, while eachcomponent's corresponding compliant socket can be batch-fabricated on aprinted wiring/waveguide board or module.

[0045] Compliant Pillar/Compliant Socket Component Sets

[0046] Reference will now be made to the figures. FIG. 1A illustrates across-sectional view of a representative embodiment of an I/Ointerconnection system 10. The I/O interconnection system 10 includes afirst structure 10 a and a second structure 10 b. The first structure 10a includes a first substrate 12 and a compliant pillar 14. The compliantpillar 14 includes a non-flat tip at the end opposite the firstsubstrate 12. The second structure 10 b includes a second substrate 20and a compliant socket 22 adapted to receive the compliant pillar 14.The embodiment shown in FIGS. 1A and 1B illustrate that the innersurface of the compliant socket has a curved or slanted surface. FIG. 1Billustrates cross-sectional views of the compliant pillar (cross sectiona-a of FIG. 1A) and the compliant socket (cross section b-b of FIG. 1A).

[0047] The first substrate 12 can include, but is not limited to,electronic and optoelectronic chips. The first substrate 12 can includeadditional components such as, but not limited to, die pads, leads,input/output components, waveguides (e.g., optical and RF), air gaps,planar waveguides, polymer waveguides, optical waveguides having opticalcoupling elements such as diffractive grating coupler and mirrorsdisposed adjacent or within the optical waveguide, photodetectors, andoptical sources such as VCSELS and LEDs.

[0048] The second substrate 20 can include, but is not limited to, aprinted wiring board, a printed wiring/waveguide board, and appropriatemating substrates. The second substrate 20 can include additionalcomponents such as, but not limited to, die pads, leads, input/outputcomponents, waveguides (e.g., optical and RF), air gaps, planarwaveguides, polymer waveguides, optical waveguides having opticalcoupling elements such as diffractive grating coupler and mirrorsdisposed adjacent or within the optical waveguide, photodetectors, andoptical sources such as VCSELS and LEDs.

[0049] In general, materials that exhibit one or more of the following,(a) process compatibility with standard microelectronic fabricationprocesses, (b) suitable mechanical strength, flexibility, anddurability, (c) sufficient lifetime and/or reliability characteristics,(d) low loss, and (e) photodefinability that can serve as the pillarmaterial and/or the compliant socket material.

[0050] In another embodiment, the pillar material and/or the compliantsocket material may need to have optical characteristics to guideoptical energy such as transparency to a particular optical wavelengthof light and/or process compatibility with other materials such that acontrast in refractive index is achieved. A reference describing polymermaterials suitable for optical waveguide applications can be found in A.R. Blythe and J. R. Vinson, Proc. 5^(th) International Symposium onPolymers for Advanced Technologies, Tokyo, Japan: pp. 601-11,August-December 2000, which is incorporated herein by reference.

[0051] In particular, the compliant pillar 14 and the compliant socket22 can be made of a low modulus material such as, but not limited to,polyimides, epoxides, polynorbornenes, polyarylene ethers, andparylenes. In particular, the low modulus materials can include, but arenot limited to, compounds such as Amoco Ultradel™ 7501, Promerus LLC's,Avatrel™ Dielectric Polymer, DuPont™ 2611, DuPont 2734, DuPont 2771, andDuPont 2555. Preferably, the compliant pillar and the compliant sockethave been fabricated by photodefinition and additional processes usingthe polymer material Avatrel 2000P from Promerus, LLC, or the like,which have shown high optical quality and high compliance.

[0052] Furthermore, the compliant pillar 14 can be fabricated to havevarying indices of refraction within different regions. For example, ifa polymer pillar is 150 μm tall, 50 μm closest to the first substrate 12can have a first index of refraction, the next 50 μm can have a secondindex of refraction, and the last 50 μm (the end opposite the firstsubstrate 12) can have a third index of refraction.

[0053] The compliant pillar 14 depicted in FIGS. 1A and 1B has a lateralcircular cross section, while the compliant socket 22 as an inner 24 andan outer 26 lateral circular cross section. The inner lateral crosssection 24 defines the area that receives the compliant pillar 14. Thus,when the first structure 12 and the second structure 20 are aligned andcoupled, the compliant socket 22, in the area defined by the innerlateral cross section 24, receives a portion of the compliant pillar 14.

[0054] Embodiments of the compliant pillar 14 can be fabricated to beflexible in the x-, y- and/or z directions. In particular, the compliantpillar 14 exhibits greater flexibility and compliance in the x-, y- axiscompared to the z-axis. However, fabrication of the compliant pillar 14on a polymer bridge (described below) can enhance compliance in thez-axis direction.

[0055] It should be noted that the compliant pillar 14 and compliantsocket 22 assist, in part, in aligning the first substrate 12 and secondsubstrate 20. In order to make a permanent mechanical interconnection, acompatible material, such as polymers and epoxies, can be depositedwithin the sockets 22 to hold the pillars 14 in place. In particular,solder and conductive adhesives can be deposited for electricalinterconnections, for example.

[0056] The cross sections of the compliant pillar 14 and the compliantsocket 22 are not limited to the lateral circular cross section shown inFIG. 1B. Also, it should be noted that the lateral cross-sectional shapeof the compliant pillar and the lateral cross-sectional shape of thesocket do not have to be the same. For example, the lateralcross-sectional shape of the compliant pillar can be substantiallyhexagonal, while the lateral cross-sectional shape of the outer portionof the compliant socket is substantially circular. Also, note that thelateral inner cross-sectional shape of the compliant socket issubstantially the same as the lateral cross-sectional shape of thecompliant pillar. In general, the compliant socket should be slightlylarger to allow easy entry of the compliant pillar into the inneropening. In addition, the inner sidewalls of the sockets may be slantedwith a positive slope to enhance pillar to board alignment.

[0057] In addition, the compliant pillar 14 can have a cross sectionsuch as, but not limited to, a polygonal cross section, a circular crosssection, and an elliptical cross section. The compliant socket 22 canhave an inner lateral cross section such as, but not limited to, apolygonal cross section, a circular cross section, and an ellipticalcross section. Likewise, the outer lateral cross section of thecompliant socket 22 can have a cross section such as, but not limitedto, a polygonal cross section, a circular cross section, and anelliptical cross section.

[0058] The non-flat tip of the compliant pillar 14 can have varioustopographies such as, but not limited to, the shapes illustrated inFIGS. 1A, 1B, and 2A through 2I. For example, the non-flat tip of thecompliant pillar 14 can be rounded (FIG. 2D), pointed (FIG. 2A), orsquared off on a portion of the tip (FIGS. 2B and 2C). In addition, thenon-flat tip can be partially slanted (FIG. 2E), have teeth cut on aportion of the tip (FIGS. 2F, 2G, and 2H), or be concave (21). Theconfigurations illustrated in FIGS. 1A, 1B, and 2A through 2I arenon-limiting and other non-flat tip configurations are included withinthe claimed subject matter. In general, the various types of tiptopography can facilitate two different functions. A tip topography mayenhance and/or assistant in making a better mechanical interconnectionbetween the socket and the pillar (i.e., the tips in FIGS. 2B and 2C).In addition, a tip topography can be used for optical interconnectionpurposes (i.e., the tips in FIGS. 2E through 2I). As such, the relativescale of the tip topography for each function can vary significantly andthe tip topography can be designed accordingly.

[0059] The compliant socket 22 can have various shapes such as, but notlimited to, the shapes illustrated in FIGS. 1A and 1B and 3A and 3B. Theslanted portion of the compliant socket 22 can include the entire innercircumference of the compliant socket 22 (FIGS. 1A and 1B), or bedivided into a plurality of segments (e.g., two segments (FIG. 3A) andfour segments (FIG. 3B)). The angle of the slope can vary depending onthe application. Also, the slope can have ridges that resist thecompliant pillar 14 from being pulled out of the compliant socket 22. Inaddition, the base of the compliant socket 22 can be made of a materialother than a polymer or directly attached to the second substrate 20 asshown in FIG. 6. The configurations illustrated in FIGS. 1A, 2B, 3A, and3B are non-limiting and other configurations are included within theclaimed subject matter.

[0060] The compliant pillar 14 can have a height from about 5 to about300 micrometers, a width of about 2 to about 150 micrometers, and alength of about 2 to about 150 micrometers. Preferably, the compliantpillar 14 can have a height from about 15 to about 150 micrometers, awidth of about 5 to about 50 micrometers, and a length of about 5 toabout 50 micrometers.

[0061] The type, size, and shape of the compliant pillar 14 andcompliant socket 22 determine the compliancy of the compliant pillar 14and the compliant socket 22. Therefore, selecting the type, size, andshape of the compliant pillar 14 and compliant socket 22 can, in part,control the amount of compliance.

[0062] In addition, the compliance of the polymer pillar is a functionof the cure temperature (e.g., such as 180 to 200° C.) and time duration(e.g., such as 1 to 4 hours) of the cure temperature. For example, thecure temperature for Avatrel 2000P is from about 180 to 200° C. for atime duration of about 1 to 4 hours. Other polymers may have curetemperatures and time durations outside of the above stated range, butone skilled in the art can adjust experimental conditions as needed. Thepolymer pillar has a lateral compliance in the range of about 2 to 20micrometers per milli-Newton. For example, compliant pillars about 100micrometers tall and having a radius of about 55 micrometers wideyielded compliance in the range from 2.5 to 5 micrometers permilli-Newton. The compliant pillar yielded this range of values becauseof the cure conditions the pillars were subjected to after fabrication.Therefore, the value of compliance can be controlled by the cureconditions. In general, “stiff” compliant pillars can be fabricatedunder high cure temperature over long cure time conditions, while ‘soft’compliant pillars can be fabricated under low cure temperature overshort (or none) cure time conditions.

[0063] In general, taller pillars yielded higher compliance. However, itshould be verified that the compliant pillars are not too “soft” in thetransverse direction. This ensures that the pillars do not “crumble”during assembly or processing.

[0064] For the purposes of illustration only, and without limitation,embodiments of the present invention will be described with particularreference to the below-described fabrication methods. Note that notevery step in the process is described with reference to the processdescribed in the figures hereinafter. Therefore, the followingfabrication processes are not intended to be an exhaustive list thatincludes every step required to fabricate the embodiments of the I/Ointerconnect system 10.

[0065]FIGS. 4A through 4F are cross-sectional views that illustrate arepresentative process for fabricating the compliant pillar 14illustrated in FIGS. 1A and 1B. FIG. 4A illustrates the substrate 16,while FIG. 4B illustrates a pillar material layer 18 disposed upon thesubstrate 16. The pillar material layer 18 can be deposited on thesubstrate 16 by methods such as, for example, spin-coating,doctor-blading, and plasma deposition. The adhesion of the pillarmaterial layer 18 to substrate 16 is such to allow later release of thedefined pillar.

[0066]FIG. 4C illustrates the addition of the hard mask 19 disposed uponthe pillar material layer 18. The hard mask 19 can be made of a maskmaterial such as, but not limited to, any material that is selective topolymer etching, such as metals and silicon dioxide, for example.Alternatively, no hard mask is necessary when the compliant pillar isphotodefined.

[0067]FIG. 4D illustrates the etching of the pillar material layer 18,which forms the compliant pillar 14. The pillar material layer 18 canalso be formed using techniques such as, for example, reactive ionetching (RIE), wet etch, and laser drilling.

[0068]FIG. 4E illustrates the removal of the hard mask 19 and theintroduction of the substrate 16 and compliant pillar 14 to the firstsubstrate 12. FIG. 4F illustrates the first substrate 12 having thecompliant pillar 14 disposed thereon. An adhesive on the first substrate12 is used to adhere to the base of the compliant pillar 14. Once thecompliant pillar 14 is introduced on the adhesive, the assembled systemis heated to improve attachment prior to release of the substrate 16.

[0069] If the material layer 18 is photosensitive, the compliant pillarcan be fabricated by exposing the material 18 in FIG. 4B through a maskto a light source with an appropriate wavelength. The mask contains thecross-sectional geometry of the compliant pillars. After exposure, theexposed material layer 18 may need a hard bake before developing. Duringdeveloping, a wet chemical agent can be used to remove the non-exposedportions (for negative tone films) of the material to leave behind thecompliant pillars (or sockets). As a result, no hard mask is needed forthe fabrication processes.

[0070]FIGS. 5A through 5F are cross-sectional views that illustrate arepresentative process for fabricating the compliant socket 22illustrated in FIGS. 1A and 1B. FIG. 5A illustrates the substrate 28,while FIG. 5B illustrates a substrate material layer 30 disposed uponthe substrate 28. The substrate material layer 30 can be deposited onthe substrate 28 by methods such as, for example, spin-coating,doctor-blading, and plasma deposition.

[0071]FIG. 5C illustrates the addition of the hard mask 32 disposed uponthe substrate material layer 30. The hard mask 32 can be made ofmaterials like those discussed above in reference to FIGS. 4A through4F. Alternatively, no hard mask 32 is necessary when the compliantsocket 22 is photodefined.

[0072]FIG. 5D illustrates the etching of the substrate material layer30, which forms the compliant socket 22. The substrate material layer 30can also be formed using techniques such as, for example, reactive ionetching (RIE), photo-definition, molding, and laser drilling. FIG. 5Eillustrates the removal of the hard mask 32 from the compliant socket22, and the introduction of the substrate 28 and compliant socket 22 tothe second substrate 20. FIG. 5F illustrates the second substrate 20having the compliant socket 22 disposed thereon.

[0073] In another representative process, the positive slope of thecompliant socket 22 can be formed by directly depositing the material onthe substrate 20 and then patterning it into the compliant socket. Next,the compliant socket 22 is heated to cause shrinkage of the sidewalls.Alternatively, RIE can be used to pattern the polymer material such thatcompliant sockets end up with slanted sidewalls. In yet anotherrepresentative process, the positive slope of the compliant socket 22can be formed by photodefinition of a positive scale photosensitivepolymer. As mentioned above, the base portion of the compliant socketcan be free of a polymer film.

[0074] Fabrication steps similar to that described above in reference toFIGS. 4A through 4F can be used if the substrate material layer 30 isphotosensitive.

[0075] Dual Optical/Electrical I/O Interconnect Component Sets

[0076]FIG. 6 illustrates a cross-sectional view of a representativeembodiment of a dual optical/electrical I/O interconnection system 40.Again, it should be noted that the figures and components are not toscale. For example, the mirrors shown may be much smaller in size thanthe planar waveguides shown. The dual optical/electrical I/Ointerconnect system 40 includes a first structure 40 a and a secondstructure 40 b. The first structure 40 a includes a first substrate 12and a dual optical/electrical I/O interconnection 41. In addition, thefirst substrate 12 includes, but is not limited to, a die pad 47, afirst waveguide 42 a, and a coupling element 46. The dualoptical/electrical I/O interconnection 41 includes a pillar 44 and anelectrical lead 48. The lead 48 can be deposited such that a metal oralloy fully encapsulates the pillar 44 except for an area for theoptical interconnection path. Otherwise, the lead 44 may cover only aportion of the sidewall, as shown in FIG. 6. The second structure 40 bincludes a second substrate 20, a socket 54, a second waveguide 42 b,and an electrical contact 56. The pillar 44 has a non-flat tip (slantedtip). The electrical lead 48 is disposed on a portion of the die pad 47and on a portion of the pillar 44 extending over the slanted area of thepillar 44. The electrical lead 48 and pillar 44 form theoptical/electrical I/O interconnect 41. The socket 54 includes a solderor other adhesive material 58 disposed therein. The socket 54 is adaptedto receive the optical/electrical I/O interconnect 41. In addition, thepillar 44 and/or the socket 54 can be fabricated from compliantmaterials that are allow the pillar and socket to be compliant. Thefirst substrate 12 and the second substrate 20 can include additionalcomponents, as described above. The lead electrical lead 48 can includematerial that is highly reflective to the optical signal wavelength.

[0077] Alternatively, without the metal lead air can be used as thewaveguide cladding because no underfill is required for the pillar 44since the pillar 44 is are laterally compliant. This enables them tocompensate for the different thermo-mechanical expansions between thechip and the board. Thus, optical/electrical I/O interconnection 41mitigate the offsets introduced due to expansion mismatches andnonplanarity. The air cladding and the resulting high index ofrefraction difference (Δn) between the core and the cladding has thebenefit of confining the optical wave and thus minimizing crosstalk. Aircladding also has two additional benefits when compared to non-aircladding in this application: 1) the pillar 44 can guide an optical wavethrough larger bends (due to large Δn), which means higher compliance,and 2) the air cladding does not impose any mechanical/physicalconstraints on the movement of the pillar 44. Thus, air waveguidecladding offers the lowest index of refraction possible and is the leastmechanically-resistant material. However, the pillar 44 may bepassivated with any cladding material, if desired.

[0078] The pillar 44 and the socket 54 can be made of materials similarto those discussed in reference to FIGS. 1A, 1B, 2A through 2I, 3A, and3B. In addition, the size and shape of the pillar 44 and compliantsocket 54 can be similar to those discussed in reference to FIGS. 1A,1B, 2A through 2I, 3A, and 3B. For example, the compliant pillar 44 canbe pointed, partially slanted, or have teeth cut on a portion of thetip. The configuration illustrated in FIG. 6 is non-limiting and othernon-flat tip configurations are included within the claimed subjectmatter.

[0079] The pillar 44 can have a height from about 5 to about 300micrometers, a width of about 2 to about 150 micrometers, and a lengthof about 2 to about 150 micrometers. Preferably, the pillar 44 can havea height from about 30 to about 150 micrometers, a width of about 5 toabout 50 micrometers, and a length of about 5 to about 50 micrometers.The socket 54 can have a height from about 5 to 30 micrometers and awidth of about 1.1 to 4 times the width of the compliant pillar.

[0080] In embodiments where the pillar 44 is made of a compliantmaterial (like those discussed in reference to FIGS. 1A, 1B, 2A through2I, 3A and 3B), the pillar 44 is flexible in the x-, y- and/or -zdirections and therefore, no underfill is needed. The fabrication of theelectrical lead 48 disposed on the pillar 44 may lower the compliancy ofthe aggregate structure. The compliance of the pillar 44 tends todecrease with the fabrication of metal on its sidewalls because themetal has a much higher stiffness than polymers and tends to plasticallydeform. The thickness of metal is preferably selected such that ityields low parasitic electrical interconnection without disturbing thehigh compliance of the intrinsic polymer pillars, and is highlyreflective to the optical wavelength of interest.

[0081] The type, size, and shape of the pillar 44 and compliant socket54 determine the compliancy of the pillar 44 and the compliant socket54. Therefore, selecting the type, size, and shape of the pillar 44 andcompliant socket 54 can, in part, control the amount of compliance.

[0082] The pillar 44 functions as a medium through which optical energytravels. As such, the pillar 44 can communicate optical energy from thefirst substrate 12 to the second substrate 20 using one or morewaveguides that may include one or more coupling elements and/or one ormore mirrors. The waveguides, coupling elements, and/or the mirrors canbe included within and/or disposed upon the first or second substrate 12and 20. As illustrated in FIG. 6, the first substrate 12 includes thefirst waveguide 42 a having the coupling element 46 disposed adjacentthe pillar 44 (as demonstrated in R. Chen, et al., “Fully EmbeddedBoard-Level Guided-Wave Optoelectronic Interconnects,” Proc. IEEE, Vol.88, pp.780-793, June 2000 incorporated herein by reference). The lead 48disposed on the pillar 44 acts as a mirror on the non-flat tip (slantedportion) of the pillar 44. The second substrate 20 includes the firstwaveguide 42 a. Therefore, optical energy can be directed into thepillar 44 via the first waveguide 42 a and the coupling element 46disposed on the first substrate 12, guided by the pillar 44, anddirected by the mirror (lead) into the second waveguide 42 b disposed onthe second substrate 20.

[0083] If the second waveguide 42 b is terminated with a mirror (R.Chen, et al., “Fully embedded board-level guided-wave optoelectronicinterconnects,” Proc. IEEE, Vol. 88, pp.780-793, June 2000, which isincorporated herein by reference) or a diffractive grating coupler (S.Schultz, et al., “Design, fabrication, and performance ofpreferential-order volume grating waveguide couplers,” Appl. Opt., vol.39, pp.1223-1232, March 2000, which is incorporated herein byreference), then the pillar 44 without a coupling element would beplaced above that terminated region of the second waveguide 42 b.

[0084] In one embodiment, the pillar does not have a slanted tip, butrather has a diffractive grating coupler disposed on the tip of thecompliant pillar. A pillar having a diffractive grating coupler disposedon the tip of the pillar can be made in a manner similar as thestructure described in FIGS. 8 and 9A through 9I and the accompanyingtext. In this case, the diffractive grating coupler can be placedadjacent to or above the optical waveguide 42 b. It should be noted thatthere is freedom with respect to the choice of the optical element to beused to mitigate the surface-normal (right-angle) bends and to itslocation (on the pillar 44 or waveguide 42 b). Moreover, the index ofrefraction of the socket material may be lower than that of thecompliant pillar waveguide.

[0085] The first waveguide 42 a and second waveguide 42 b can be definedthrough multiple fabrication processes such as, but not limited to,photo-definition, wet chemical etching, dry plasma etching,thermally-induced refractive index gradients, and ion implantation. Inaddition, the first waveguide 42 a and second waveguide 42 b can havegeometries such as, for example, raised strip geometry, buried geometry,and rib geometry.

[0086] The coupling element 46 can include mirrors, planar (or volume)grating couplers, evanescent couplers, surface-relief grating couplers,and total internal reflection couplers, for example. More specifically,when the coupling element 46 is a volume grating coupler, the couplingmaterial can be laminated or spin-coated onto the appropriate surface.In particular, a laminated volume grating coupler can be formed byholographic exposure of the grating region following lamination of thegrating material. Alternatively, the laminated volume grating couplercan be formed by holographic exposure prior to lamination of the gratingmaterial. In the case where the coupling element 46 is to be formedinside of the compliant pillar waveguide, the compliant pillar waveguideand coupler can be composed of separate materials. Additional detailsregarding grating couplers can be found in U.S. Pat. No. 6,285,813,which is incorporated herein by reference.

[0087] If the coupling element 46 is a grating coupler, then the gratingcoupler material includes materials such as, for example, polymermaterials, silver halide photographic emulsions, photoresists such asdichromated gelatin, photopolymers such as polymethyl methacrylate(PMMA) or Dupont™ HRF photopolymer films, for example, thermoplasticmaterials, photochromic materials such as crystals, glasses or organicsubstrates, photodichroic materials, and photorefractive crystals suchas lithium niobate, for example. These materials have thecharacteristics of creating a refractive index modulation through avariety of mechanisms, all of which result in the creation of a phase orabsorption or mixed grating. Other suitable materials are described inT. K. Gaylord and M. G. Moharam, Proc. IEEE, vol. 73, pp. 894-937, May1985, which is herein incorporated by reference. The fabrication of agrating coupler is preferred to be done on the pillar, and thus at thewafer-level, where nano-lithography is readily available. To fabricatesuch a device on the printed wiring/waveguide board would potentially beexpensive.

[0088] An additional feature of the pillar 44 is that portions of thepillar 44 not bound by the socket 54 are surrounded by air, which actsas an air-gap cladding layer. The advantages of the air-cladding in thisapplication are described above. It should be pointed out that some ofthe material requirements for conventional optical interconnects do notnecessarily apply to the pillar 44. For example, the materials are notrestricted to ultra-low absorption optical materials due to the shortheight (below 300 micrometers) of the compliant pillar.

[0089] The socket 54 shown in FIG. 6 includes a solder material 58 toassist in the attachment of the lead to the socket 54. The soldermaterial 58 can be a material such as, but not limited to, lead andlead-free solder such as tin-lead and tin-copper-silver alloy solders.In addition, conductive adhesives can also be used as the soldermaterial.

[0090] The die pad 47 is assumed to already exist on the die that thepillars 44 are fabricated on. Otherwise, the die pad 47 can be depositedupon the surface of the first substrate 12 using techniques such as, forexample, sputtering, evaporation, electron-beam deposition,electroplating, electro-less plating, and displacement reactions.

[0091] The geometry of the leads 48 that can be used in embodiments ofthe present invention is not limited to that shown in FIG. 6. Instead,various lead 48 geometries can provide compliance consistent with thescope of the present invention. Additional steps can be performed tofabricate an attachment or contact on the end portion of the electricallead. This contact (not shown) can include a variety of items designedto contact or attach to a pad or point on another substrate. Thesecontacts can be, for example, a solder bump or a conductive adhesive.

[0092] The lead 48 can be an electrical lead or a radio frequency lead.The lead 48 can be fabricated of one or more layers of metals, metalcomposites, conductive adhesives, or combinations thereof, appropriatefor the electrical/optical I/O interconnect system 40. The metals andmetal composites include, but are not limited to, gold, gold alloys,copper, and copper alloys. The lead 48 can be fabricated bymonolithically electroplating the selected metal or metal composite ontothe first substrate.

[0093] The lead 48 can have a thickness that ranges from about 0.1 toabout 30 micrometers, and preferably from about 0.1 to about 5micrometers. The preferred embodiment of the lead 48 has a thickness ofabout 2 micrometers. The lead 48 can have lengths that range from about2 to about 300 micrometers, and preferably from about 30 to about 150micrometers. The lead 48 can have a width that ranges from about 1 toabout 100 micrometers, and preferably from about 2 to about 40micrometers. The lead 48 can have a height that ranges from about 10 toabout 300 micrometers, and preferably from about 30 to about 150micrometers. As mentioned above, the lead 48 can be disposed over alarge portion of the pillar 44 (e.g., except the area used for opticaltransmission), so that the dimensions of the lead 48 may be very similarto that of the pillar 44.

[0094] For the purposes of illustration only, and without limitation,embodiments of the present invention will be described with particularreference to the below-described fabrication methods. Note that notevery step in the process is described with reference to the processdescribed in the figures hereinafter. Therefore, the followingfabrication processes are not intended to be an exhaustive list thatincludes every step required to fabricate the embodiments of theoptical/electrical I/O interconnect system 40.

[0095]FIGS. 7A through 7F are cross-sectional views that illustrate arepresentative process for fabricating the pillar 44 illustrated in FIG.6. FIG. 7A illustrates the molding substrate 60, while FIG. 7Billustrates a pillar material layer 50 disposed upon the moldingsubstrate 60 as well as the addition of a hard mask 52. The pillarmaterial layer 50 can be deposited on the molding substrate 60 bymethods such as, for example, spin-coating, doctor-blading, and plasmadeposition. The hard mask 52 can be made of a mask material such as, butnot limited to, any material that is selective to polymer etching, suchas metals and silicon dioxide, for example. Alternatively, no hard maskis necessary when the compliant pillar is photodefined.

[0096]FIG. 7C illustrates the etching of the pillar material layer 50,which forms the pillar 44. The pillar material layer 50 can also beformed using techniques such as, for example, reactive ion etching(RIE), wet etch, and laser drilling.

[0097]FIG. 7D illustrates the removal of the mask 52 and theintroduction of the molding substrate 60 and pillar 44 to the firstsubstrate 12. FIG. 7E illustrates the first substrate 12 having thecompliant pillar 44 disposed thereon. FIG. 7F illustrates the formationof the lead 48 on a portion of the die pad 47 and the pillar 44.

[0098] If the material layer 50 is photosensitive, the compliant pillarcan be fabricated by exposing the material 16 in FIG. 3B through a maskto a light source with an appropriate wavelength. The mask contains thecross-sectional geometry of the compliant pillars. After exposure, theexposed material layer 50 may need a hard bake before developing. Duringdeveloping, a wet chemical agent can be used to remove the non-exposedportions (for negative tone films) of the material to leave behind thecompliant pillars (or sockets). As a result, no hard mask is needed forthe fabrication processes.

[0099] The compliant socket can be fabricated in a manner similar to themethod described in FIGS. 4A through 4F and the corresponding text.

[0100] Modified-Tip Topographry for Pillars

[0101] In general, the tip of a pillar can be modified from a flat tipto a non-flat tip by forming the pillars as described above in FIGS. 1A,1B, and 2A through 2I. In addition, the tip of the pillar can bemodified by forming a coupling element, a mirror, or lens, on the tip ofthe pillar. The modified-tip topography can be used to alter thedirection (e.g., direct into a different direction or directions and/orfocus the optical energy) of optical energy out of or into the compliantpillar. For example, a concave mirror disposed on the pillar (not shown)can couple optical energy from the polymer pillar into a slab waveguidein the event that the cross-sectional size and geometry of the pillarand waveguide are not the same. Alternatively, a cylindrical polymerpillar can be used as a converging lens to focus optical energy into acoupling element on a slab waveguide (not shown). The mirror and lenscould be used together or separately to focus optical energy.

[0102]FIGS. 8, 11A, and 11B illustrate two representative embodiments ofpillars having modified-tip topography. FIG. 6 is also representative ofan embodiment of a pillar with modified-tip topography (i.e., with onemirror instead of two as in FIG. 8). These embodiments are non-limitingand other modified-tip topographies are included within the claimedsubject matter.

[0103]FIG. 8 illustrates a cross-sectional view of a representativeembodiment of a modified-tip structure 70 with a modified-tip pillar 71.The structure 70 includes, but is not limited to, substrate 72, a pillar74, and mirrors 76 disposed in the tip of the pillar 74. Themodified-tip pillar 71 includes the pillar 74 and the mirrors 76. Thepillar 74 acts as an optical waveguide similar to the pillars 14 and 44described above.

[0104] The substrate 72 can include, but is not limited to, thecomponents described above in reference to the first and secondsubstrates 12 and 20. The pillar 74 can be made of similar materials asthe pillars described above in reference to FIGS. 1A, 1B, and 2A through2I. In addition, the type, size, and shape of the pillar 74 can besimilar to the pillars described above in reference to FIGS. 1A, 1B, and2A through 2I.

[0105] The mirrors 76 can be fabricated from mirror material such as,but not limited to, a slanted polymer coated with a metal film, andunmetallized slants for total internal reflection mirrors.

[0106] The mirrors 76 illustrated in FIG. 8 can direct optical energy intwo directions. However, additional embodiments can include mirrors thatdirect optical energy in one or more directions.

[0107] For the purposes of illustration only, and without limitation,embodiments of the present invention will be described with particularreference to the below-described fabrication methods. Note that notevery step in the process is described with reference to the processdescribed in the figures hereinafter. Therefore, the followingfabrication processes are not intended to be an exhaustive list thatincludes every step required to fabricate the embodiments of themodified-tip structure 70.

[0108]FIGS. 9A through 9J are cross-sectional views that illustrate arepresentative process for fabricating the modified-tip structure 70having the modified-tip pillar 71 illustrated in FIG. 8. FIG. 9Aillustrates the substrate 72, while FIG. 9B illustrates a pillarmaterial layer 78 disposed upon the substrate 72. The pillar material issimilar to the pillar material described above. The pillar materiallayer 78 can be deposited on the substrate 72 by methods such as, forexample, spin-coating, doctor-blading, and plasma deposition.

[0109]FIG. 9C illustrates the addition of a hard mask 80 to the pillarmaterial layer 78. The hard mask 80 can be made of a mask material suchas, but not limited to, any material that is selective to polymeretching, such as metals and silicon dioxide, for example. Alternatively,no hard mask is necessary when the compliant pillar is photodefined.

[0110]FIG. 9D illustrates the etching of the pillar material layer 78,which forms the unmodified pillar 82. The pillar material layer 78 canalso be formed using techniques such as, for example, reactive ionetching (RIE), wet etch, and laser drilling.

[0111]FIG. 9E illustrates the removal of the hard mask 80, while FIG. 9Fillustrates the introduction of the substrate 72 and unmodified pillar82 to the impression mold 84. The impression mold can be used to indentor modify the tip of the unmodified pillar 82. The impression mold cantake various shapes (i.e., sawtooth cutouts) and is dependent upon thecomponent that is going to be formed on the tip of the pillar.

[0112] Other methods for indenting or modifying the tip of theunmodified pillar 82 may include, but is not limited to, thermallycuring the unmodified pillar 82 while the mold pattern is impressedagainst the unmodified pillar 82 or heating and pressing the impressionmold 84 on the pillar to cause local heating and local deformation.These processes can be performed after the polymer film application andsoftbake (FIG. 9B). Another method for indenting or modifying theunmodified pillar 82 may include, but is not limited to, spin coatingthe polymer, 70% soft baking, molding the film while finishing the softbake process step, removing the mold, and then photoimaging. The slantedsurfaces on the tip of the pillar 74 can also be formed by reactive ionetching (RIE). A reference describing RIE of slanted surfaces can befound in G. Boyd et al. “Directional Reactive Ion Etching at ObliqueAngles,” Appl. Phys. Lett., vol. 36, no., 7, pp. 583-585, April 1980,which is incorporated herein by reference.

[0113]FIG. 9G illustrates the pillar 74 after the tip has been modified.FIG. 9H illustrates a mirror layer 82 disposed upon the substrate 72 andpillar 74. The mirror layer 82 can be deposited on the substrate 16 bymethods such as, for example, sputtering, or electron beam evaporation.

[0114]FIG. 9I illustrates the addition of a hard mask 88 over a portionof the mirror layer 82 disposed over the modified-tip of the pillar 74.The hard mask 88 can be made of a mask material such as, but not limitedto, any material that is selective to polymer etching, such as metalsand silicon dioxide, for example. Alternatively, no hard mask isnecessary when the compliant pillar is photodefined.

[0115]FIG. 9J illustrates the mirror layer 82 etched away, which formsthe mirrors 76 on the tip of the pillar 74. The mirror layer 82 can alsobe etched using techniques such as, for example, reactive ion etching(RIE), wet etch, and laser drilling. It should be noted that thefabrication process described in FIGS. 7A through 7G could be used tofabricate modified-tip structure 70.

[0116]FIG. 10 illustrates the path that optical energy can travel usingthe modified-tip pillar 71 shown in FIG. 8. Initially, the opticalenergy travels from two directions through waveguides 94 a and 94 bdisposed on substrate 92. Upon encountering the mirrors 76, the opticalenergy is diverted into the modified-tip pillar 71. FIG. 10 is only anillustrative example of how the modified-tip pillar 71 can be used. Theimportance of such an interconnection depends on the overallarchitecture of the system. It may be necessary to have two chipscommunicating to the same location on a third chip, for example. FIGS.11A and 11B illustrate another type of modified-tip pillar in which themodified-tip structures 100 a and 100 b have lenses 106 a and 106 bdisposed on the pillar 104. The lenses 106 a and 106 b can be used tofocus optical energy into or out of the pillar 104. The modified-tipstructures 100 a and 100 b include, but are not limited to, a substrate102, a pillar 104, and a lens 106 a and 106 b, respectively. The pillar104 acts as an optical waveguide similar to the pillars 14 and 44described above.

[0117] The substrate 102 can include, but is not limited to, thecomponents described above in reference to the first and secondsubstrates 12 and 20. The pillar 104 can be made of similar pillarmaterials as the pillars described above in reference to FIGS. 1A, 1B,and 2A through 2I. In addition, the type, size, and shape of the pillar104 can be similar to the pillars described above in reference to FIGS.1A, 1B, and 2A through 2I.

[0118] The lens can take the form of that shown in FIG. 11A or 11B. Thelenses 106 a and 106 b can be fabricated from lens materials such as,but not limited to, polymers that exhibit good adhesion and the desiredoptical properties. The lenses 106 a and 106 b can be formed by dippingthe pillar 104 into a lens material layer (which is still in semi-liquidform) at room temperature or above, so that a portion of the lensmaterial is disposed onto the tip of the pillar 104 and forms a lens.

[0119] For the purposes of illustration only, and without limitation,embodiments of the present invention will be described with particularreference to the below-described fabrication methods. Note that notevery step in the process is described with reference to the processdescribed in the figures hereinafter. Therefore, the followingfabrication processes are not intended to be an exhaustive list thatincludes every step required to fabricate the embodiments of themodified-tip structure 100 a.

[0120]FIGS. 12A through 12F are cross-sectional views that illustrate arepresentative process for fabricating the modified-tip structure 100 aillustrated in FIGS. 11A and 11B. FIG. 12A illustrates the substrate102, while FIG. 12B illustrates a pillar material layer 108 disposedupon the substrate 102. The pillar material layer 108 can be depositedon the substrate 102 by methods such as, for example, spin-coating,doctor-blading, and plasma deposition.

[0121]FIG. 12C illustrates the addition of a hard mask 110 to the pillarmaterial layer 108. The hard mask 110 can be made of a mask materialsuch as, but not limited to, any material that is selective to polymeretching, such as metals and silicon dioxide, for example. Alternatively,no hard mask is necessary when the compliant pillar is photodefined.

[0122]FIG. 12D illustrates how the pillar material layer 108 is etchedto form the pillar 104. The pillar 104 can also be formed usingtechniques such as, for example, reactive ion etching (RIE), wet etch,and laser drilling.

[0123]FIG. 12E illustrates the removal of the mask 110 and illustratesthe introduction of the substrate 102 and pillar 104 to the lensmaterial 114 disposed onto substrate 112. The lens material 114 is inliquid form and can be heated to temperatures ranging from roomtemperature to 150° C., for example. Thus, when the substrate 102 andpillar 104 are dipped into the lens material 114, the lens 106 a formson the tip of the pillar 104 as a spherelike ball, due to surfacetension. FIG. 12F illustrates the modified-tip pillar 101 a after thelens is disposed onto the pillar tip. Disposing of the lens onto thepillar tip by this process allows for different indices of refraction aswell as the same index of refraction between the pillar and lens.

[0124] Alternatively, a pillar 104 with concave/convex tip may befabricated by locally heating the tips of the polymer pillar 104 tocause local melting. Then, removing the heat source allows the moltedpolymer to cool, resolidify, and remain spherical. Such a process alsoallows for batch fabrication.

[0125] Furthermore, if the lens material 114 is photosensitive, theremoval of the lens from some pillars while leaving the others intact ina batch process can be facilitated by using a mask. A mask can also beused to modify the shape of the lens or fabricate surface reliefdiffractive elements on the lenses.

[0126]FIG. 13 illustrates the path that optical energy can travel usingthe modified-tip pillar 101 a shown in FIG. 11A. Initially, the opticalenergy travels through the pillar 104. Upon encountering the lens 106 a,the optical energy is focused onto a component 118 (e.g., a detector,waveguide, or coupling element) disposed on substrate 116. FIG. 14 isonly an illustrative example of how the modified-tip pillar 101 a can beused. For example, the optical energy could also be focused on acomponent (e.g., a detector, waveguide, or coupling element) disposedunder the surface of the substrate 116. Further, a compliant socket canbe disposed on the substrate 116, the compliant socket being analogousto the compliant socket 22 in FIGS. 1A and 1B. The compliant socket cancould allow z-axis alignment of the modified-tip pillar 101 a and theburied component in the substrate 116.

[0127] Polymer Bridge

[0128] In general, polymer bridges can be used to add z-axis complianceby disposing one or more compliant pillars, the optical/electrical I/Ointerconnects, the lens/waveguide optical pillars, and/or thecorresponding compliant sockets, upon the polymer bridge. The polymerbridge includes a polymer region that spans across an area that does nothave any material disposed thereunder.

[0129]FIG. 14A illustrates a cross-sectional view of a polymer bridge124 a, while FIG. 14B illustrates the cross-section of the polymerbridge 124 a with respect to the a-a cross-section. The polymer bridgestructure 120 a includes a substrate 122, a polymer bridge 124 a, and anunbound air-gap region 126 a. The unbound air-gap region is not enclosedby the polymer bridge 124 a.

[0130]FIGS. 15A through 15C illustrate cross-sectional views ofadditional representative embodiments of polymer bridges 120 b . . . 120d. The configuration of the polymer bridge can include additionalconfigurations to those described in FIGS. 14A and 14B and 15A through15C.

[0131] The substrate 122 can include, but is not limited to, thecomponents described above in reference to the first and secondsubstrates 12 and 20. The polymer bridge material can be made ofmaterials such as, but not limited to, the materials discussed inreference to FIGS. 1A, 1B, 2A through 2I, 3A and 3B for the pillars andsockets.

[0132] The unbound air-gap region 126 a, 126 b, 126 c, or 126 d can beformed by the removal (e.g., decomposition or removal of photoresist) ofa sacrificial layer from the area in which the unbound air-gap region126 is to be located. The unbound air-gap region 126 occupies a spacebounded, in part, by the polymer bridge 124 and the substrate 122.

[0133] Generally, during the fabrication process, the sacrificial layeris deposited onto the substrate 122. Thereafter, the polymer materiallayer 124 is deposited over a portion of the sacrificial layer.Subsequently, the sacrificial layer is removed forming the unboundair-gap region 126 a. The processes for depositing and removing thesacrificial layer are discussed in more detail hereinafter.

[0134] The sacrificial layer can be polymers that have a decompositiontemperature less than the decomposition or degradation temperature ofthe polymer bridge material. Examples of the sacrificial layer includecompounds such as, but not limited to, polynorbornenes, polycarbonates,polyethers, and polyesters. More specifically the sacrificial layerincludes compounds such as BF Goodrich Unity™ 400, polypropylenecarbonate, polyethylene carbonate, and polynorborene carbonate. Thesacrificial layer may also contain photosensitive compounds, which areadditives for patterning or decomposition. The sacrificial material mayinclude photoresists or metals.

[0135] The sacrificial layer can be deposited using techniques such as,for example, spin coating, doctor-blading, sputtering, lamination,screen or stencil-printing, melt dispensing, chemical vapor deposition(CVD), and plasma based deposition systems.

[0136] The height of the unbound air-gap region 126 a can range fromabout 5 to 80 micrometers. The dimensions of the polymer bridge 124 acan range from about 5 to 500 micrometers in length, about 3 to 30micrometers in thickness, and about 5 to 500 micrometers in width.

[0137] For the purposes of illustration only, and without limitation,embodiments of the present invention will be described with particularreference to the below-described fabrication methods. Note that notevery step in the process is described with reference to the processdescribed in the figures hereinafter. Therefore, the followingfabrication processes are not intended to be an exhaustive list thatincludes every step required to fabricate the embodiments of the polymerbridge structure 120 a.

[0138]FIGS. 16A through 16D are cross-sectional views that illustrate arepresentative process for fabricating the exemplary polymer bridge 124a illustrated in FIGS. 14A and 14B. FIG. 16A illustrates the substrate122, while FIG. 16B illustrates the sacrificial material layer 128disposed upon the substrate 122. The sacrificial material layer 128 canbe deposited on the substrate 122 by methods such as, for example,spin-coating, doctor-blading, and plasma deposition.

[0139]FIG. 12C illustrates the addition of a polymer bridge 124 over thesacrificial material layer 128. The polymer bridge 124 a can bedeposited on the substrate 122 by methods such as, for example,spin-coating, doctor-blading, and plasma deposition.

[0140]FIG. 16D illustrates the removal of the sacrificial material layer128, which forms the unbound air-gap region 126 a, so that the polymerbridge is positioned above the substrate 122.

[0141] L-Shaped Pillars

[0142] In general, L-shaped pillars can be used as interconnects. Forexample, the lateral portion of the L-shaped pillar extending from achip can be directly embedded in a printed wiring board and couple to awaveguide or source embedded in the board.

[0143]FIG. 17A illustrates a cross-sectional view of an L-shaped pillar144 a. The L-shaped structure 140 a includes, but is not limited to, afirst substrate 12 a and L-shaped pillar 144 a. FIG. 17B illustrates across-sectional view an alternate embodiment of an L-shaped pillar 144 bwith a mirror 146 b fabricated at the corner of the L-shaped pillar 144b. The first substrates 12 a and 12 b can include additional components,as described above. The L-shaped pillars 144 a and 144 b can befabricated from materials, including but not limited to, photosensitivepolymers. The photosensitive polymers can include, but are not limitedto, the materials discussed in reference to FIGS. 1A, 1B, 2A through 2I,3A and 3B for the pillars and sockets. The mirror 146 b can be made ofmaterials such as, but not limited to, metals used for simple totalinternal reflection. FIGS. 18A and 18B illustrate representative the topviews of L-shaped pillars. Alternate embodiments (not shown) couldresemble helix-like polymer interconnections or pillars terminated witha circular disk rather than a lateral polymer channel.

[0144] For the purposes of illustration only, and without limitation,embodiments of the present invention will be described with particularreference to the below-described fabrication methods. Note that notevery step in the process is described with reference to the processdescribed in the figures hereinafter. Therefore, the followingfabrication processes are not intended to be an exhaustive list thatincludes every step required to fabricate the embodiments of theL-shaped pillar 144 a.

[0145]FIGS. 19A and 19E are cross-sectional views that illustrate arepresentative process for fabricating the L-shaped pillar 144 aillustrated in FIG. 17A. FIG. 19A illustrates photosensitive polymermaterial 148 disposed upon the substrate 12 a. The photosensitivepolymer material 148 can be deposited on the substrate 12 a by methodssuch as, for example, spin coating, doctor-blading, and plasmadeposition.

[0146]FIG. 19B illustrates the addition of a hard mask 150 to thephotosensitive material 148. The hard mask is very reflective to thewavelength that the polymer is sensitive to and also adheres well to thepolymer.

[0147]FIG. 19C illustrates the addition of another layer ofphotosensitive polymer material 148 and another hard mask 150. FIG. 19Dillustrates the exposure of the photosensitive polymer material 148 atthe portions defining the L-shaped pillar 144 a.

[0148]FIG. 19E illustrates the removal of the photosensitive material148 and the hard mask 150. This can be performed by first developing thetop polymer layer 148, then removing the hard mask 150, and then finallydeveloping the bottom most polymer film 148. Removal of thephotosensitive material can be facilitated by wet etch. Mask 150 isreflective to the wavelength used to photoimage the polymer, so that thepolymer under the lowest most layer of the mask 150 is not exposed.

[0149] It should be emphasized that the above-described embodiments ofthe present invention are merely possible examples of implementations,and are merely set forth for a clear understanding of the principles ofthe invention. For example, the compliant pillars can be fabricated ofmultiple materials. The compliant pillars can also be used without thecompliant sockets being on the board. In addition, the compliant socketscan be interconnected to other non-pillar like structures. Therefore,many variations and modifications may be made to the above-describedembodiment(s) of the invention without departing substantially from thespirit and principles disclosed herein. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

Therefore, having thus described the invention, at least the followingis claimed:
 1. An input/output (I/O) interconnect system, comprising: afirst substrate having at least one compliant pillar transverselyextending from the first substrate, wherein the compliant pillarcomprises a first material, and wherein the compliant pillar includes anon-flat tip at the end opposite the first substrate.
 2. The I/Ointerconnect system of claim 1, wherein the first material comprises alow modulus material selected from polyimides, epoxides,polynorbornenes, polyarylene ethers, and parylenes.
 3. The I/Ointerconnect system of claim 1, wherein the compliant pillar has aheight of about 15 to 300 micrometers.
 4. The I/O interconnect system ofclaim 1, wherein the compliant pillar has a length of about 2 to 55micrometers and a width of about 2 to 55 micrometers.
 5. The I/Ointerconnect system of claim 1, wherein the first substrate has fromabout 10 compliant pillars to about 500,000 compliant pillars percentimeter squared of the first substrate.
 6. The I/O interconnectsystem of claim 1, further comprising: a second substrate having atleast one compliant socket adapted to receive a compliant pillar,wherein the compliant socket comprises a second material, wherein thecompliant socket includes a non-flat top surface at the end opposite thesecond substrate.
 7. The I/O interconnect system of claim 6, wherein thesecond material comprises a low modulus material selected frompolyimides, epoxides, polynorbornenes, polyarylene ethers, andparylenes.
 8. The I/O interconnect system of claim 6, wherein thecompliant socket has a height of about 5 to 30 micrometers.
 9. The I/Ointerconnect system of claim 6, wherein the compliant socket includes amaterial that secures the compliant pillar to the compliant socket. 10.The I/O interconnect system of claim 1, wherein the compliant pillar isused as a transverse waveguide that is substantially perpendicular tothe first substrate.
 11. The I/O interconnect system of claim 10,further comprising an element selected from a diffractive gratingcoupler disposed on the compliant pillar and a mirror disposed on thecompliant pillar.
 12. The I/O interconnect system of claim 11, whereinthe coupling element is selected from a volume grating coupling elementand a surface relief grating coupling element.
 13. The I/O interconnectsystem of claim 6, further comprising an element selected from adiffractive grating coupler disposed within the second substrate and amirror disposed within the second substrate.
 14. The I/O interconnectsystem of claim 7, wherein the second substrate has from about 10compliant sockets to about 100,000 compliant sockets per centimetersquared of the second substrate.
 15. The I/O interconnect system ofclaim 1, further comprising a lead disposed upon a portion of thecompliant pillar.
 16. The I/O interconnect system of claim 15, whereinthe lead is a radio frequency lead.
 17. The I/O interconnect system ofclaim 15, wherein the lead is an electrical lead.
 18. The I/Ointerconnect system of claim 16, wherein the first substrate has fromabout 10 compliant pillars to about 500,000 compliant pillars percentimeter squared of the first substrate.
 19. The I/O interconnectsystem of claim 17, wherein the first substrate has from about 10compliant pillars to about 100,000 compliant pillars per centimetersquared of the first substrate.
 20. A dual-mode optical/electricalinput/output (I/O) interconnect system, comprising: a first substratehaving at least one optical/electrical I/O interconnect that includes apillar transversely extending from the first substrate, wherein thepillar comprises a first material, the first material is opticallyconductive, and the pillar includes a lead disposed over a portion ofthe pillar extending from the base of the pillar on the first substrateto the end opposite the first substrate.
 21. The I/O interconnect systemof claim 20, wherein the pillar is a compliant pillar.
 22. The I/Ointerconnect system of claim 20, further comprising: a second substratehaving at least one socket adapted to receive the pillar and the lead,wherein the socket comprises a second material, wherein the secondsubstrate includes a lead contact that communicatively connects thefirst substrate and the second substrate through the lead, wherein thesecond substrate includes an optical contact that communicativelyconnects the first substrate and the second substrate through thepillar.
 23. The I/O interconnect system of claim 22, wherein the secondmaterial comprises a low modulus material selected from polyimides,epoxides, polynorbornenes, polyarylene ethers, and parylenes.
 24. TheI/O interconnect system of claim 22, wherein the socket is a compliantsocket.
 25. The I/O interconnect system of claim 22, wherein the pillarincludes a non-flat tip at an end opposite the first substrate.
 26. TheI/O interconnect system of claim 22, wherein the first materialcomprises a low modulus material selected from polyimides, epoxides,polynorbornenes, polyarylene ethers, and parylenes.
 27. The I/Ointerconnect system of claim 22, wherein the first substrate has fromabout 10 to about 100,000 optical/electrical I/O interconnects percentimeter squared of the first substrate.
 28. The I/O interconnectsystem of claim 22, further comprising an element disposed on an end ofthe pillar opposite the first substrate, the element selected from adiffractive grating coupler and a mirror.
 29. The I/O interconnectsystem of claim 28, wherein the diffractive grating coupler is selectedfrom a volume grating coupling element and a surface relief gratingcoupling element.
 30. A method for forming a device comprising:providing a first substrate having at least one optical/electrical I/Ointerconnect that includes a pillar transversely extending from thefirst substrate, wherein the pillar comprises of a first material, thefirst material is optically conductive, and the pillar includes a leaddisposed over a portion of the pillar extending from the base of thepillar on the first substrate to the end opposite the first substrate;providing a second substrate having at least one socket adapted toreceive the optical/electrical I/O interconnect, wherein the socketcomprises a second material, wherein the second substrate includes alead contact that communicatively connects the first substrate and thesecond substrate through the lead, wherein the second substrate includesan optical contact that communicatively connects the first substrate andthe second substrate through the pillar; and causing the socket toreceive a portion of the optical/electrical I/O interconnect.
 31. Amethod of aligning substrates, comprising: providing a first substratehaving at least one optical/electrical I/O interconnect that includes apillar transversely extending from the first substrate, wherein thepillar comprises of a first material, the first material is opticallyconductive, and the pillar includes a lead disposed over a portion ofthe pillar extending from the base of the pillar on the first substrateto the end opposite the first substrate; providing a second substratehaving at least one socket adapted to receive the optical/electrical I/Ointerconnect, wherein the socket comprises a second material, whereinthe second substrate includes a lead contact that communicativelyconnects the first substrate and the second substrate through the lead,wherein the second substrate includes an optical contact thatcommunicatively connects the first substrate and the second substratethrough the pillar; maintaining optical alignment between the firstsubstrate and the second substrate using the optical/electrical I/Ointerconnect and the socket; and maintaining electrical interconnectionbetween the first substrate and the second substrate using theoptical/electrical I/O interconnect and the socket.
 32. A method ofdirecting optical energy and electrical energy, comprising: providing afirst substrate having at least one optical/electrical I/O interconnectthat includes a pillar transversely extending from the first substrate,wherein the pillar comprises of a first material, the first material isoptically conductive, and the pillar includes a lead disposed over aportion of the pillar extending from the base of the pillar on the firstsubstrate to the end opposite the first substrate; providing a secondsubstrate having a socket adapted to receive the optical/electrical I/Ointerconnect, wherein the socket comprises a second material, whereinthe second substrate includes a lead contact that communicativelyconnects the first substrate and the second substrate through the lead,wherein the second substrate includes at least one optical contact thatcommunicatively connects the first substrate and the second substratethrough the pillar; communicating optical energy between the pillar ofthe first substrate and the optical contact of the second substrate; andcommunicating electrical energy between the lead of the first substrateand the lead contact of the second substrate.
 33. A method forfabricating a device having at least one compliant pillar comprising:providing a substrate; disposing a material onto at least one portion ofthe substrate; and removing portions of the material to form at leastone pillar on the substrate having smooth sidewalls that are configuredat a substantially right angle with respect to the substrate.
 34. Themethod of claim 33, further comprising: forming at least one lead on aportion of the compliant pillar, wherein the lead extends from the baseof the pillar on the substrate to the end opposite the substrate. 35.The method of claim 33, wherein the pillar includes a non-flat tip atthe end opposite the substrate.
 36. The method of claim 33, wherein thematerial comprises a low modulus material selected from polyimides,epoxides, polynorbornenes, polyarylene ethers, and parylenes.
 37. Themethod of claim 33, further comprising: forming an coupling element onthe pillar.
 38. The method of claim 33, further comprising: formingabout 10 to about 100,000 pillars per centimeter squared on thesubstrate.
 39. An input/output (I/O) interconnect, comprising: asubstrate having at least one compliant pillar transversely extendingfrom the first substrate, wherein the compliant pillar comprises a firstmaterial being optically conductive, wherein the compliant pillarincludes a lens disposed at the end opposite the first substrate,wherein the lens being adapted to focus optical energy.
 40. A device,comprising: a substrate including an overcoat polymer layer, asacrificial polymer layer disposed at a first location between thesubstrate and the overcoat polymer layer, wherein the sacrificialpolymer layer can be removed from the first location to form a compliantpolymer bridge.
 41. The device of claim 40, further comprising a pillardisposed on the compliant bridge.
 42. An input/output (I/O) interconnectsystem, comprising: a first substrate having an L-shaped compliantpillar transversely extending from the first substrate, wherein thecompliant pillar comprises a first material.